This invention relates to a sequence control system, or more in particular to an apparatus for automatically diagnosing a fault of devices to be controlled.
Conventionally, relay logic circuits, PLC (programmable logic controllers), or the like are used as sequence control systems. These sequence control systems are used for such applications as control of production processes. The production processes include at least one production line or usually several production lines. Although the sequence control system is ordinarily installed in the vicinity of such production lines, it has a high reliability in operation because of the location thereof in an environment protected from direct interference by an article in the production lines, sparks, or water. However, external sensors, such as limit switches, which supply the control system with input signals indicating the operational status, are installed within the related production lines per se and therefore liable to be disabled by impact with an article in production, sparks or water. A mis-operation of any one of such external sensors makes normal sequence control operation impossible, resulting in a fault in the system. In the case of a large-scale system including as many as several thousands of such external sensors which generate the input signals applied to the sequence control system, it is almost impossible to find the sensor which is faulty. A fault in even only one of the sensors places the whole system out of order.
Several types of method/apparatus for diagnosing an abnormal condition of the sensors which supply the sequence control system with input signals indicating the operating status of the object to be controlled have so far been suggested. They are known as (1) the process delay detection method, (2) the prohibited pattern checking method, and (3) the sequence circuit condition display method.
In the process delay detection method of (1), a time chart by which sequence steps are normally followed is stored as a reference time chart. The actual operational status is compared with the reference time chart, and if the timing difference between them exceeds a certain limit, a warning is given against an abnormal operation. The disadvantage of this method is that although the occurrence of a fault is detected, the sequence step where the erroneous condition exists or the fault has occurred can not be identified.
The prohibited pattern checking method of (2) is for checking whether or not a condition which should never occur under normal conditions has occurred. According to this method, a pattern which should never occur or a prohibited pattern is stored in advance, and a check is made to see whether or not such a pattern of condition exists for fault diagnosis. In spite of the advantage that once a prohibited pattern is present, the external sensor in which there is a problem is immediately identified, the probability of occurrence of a fault belonging to such a prohibited pattern is very small. The use of this method alone, therefore, is not very effective in fault diagnosis.
In the sequence circuit condition display method of (3), upon the occurrence of a fault, the status of suspicious sequence circuits (opened or closed conditions of contacts) included in the circuits is displayed on a circuit diagram in order to find a faulty part by presumption. The shortcoming of this method is the requirement of human presumption of the problem part.